A field device or a field device apparatus is used when performing a measuring task to determine a physical measurement variable or measurement value. The physical measurement variable can be for example a running time or a transit time, a distance, pressure, a flow rate, a flow or a temperature. From the measured physical measurement variable it is possible to derive a measurement variable that is of interest. The measurement variable of interest may be for example a fill level in a container which is determined from a physical signal running time. A radar signal can be used to determine the signal running time.
The field device often needs to be used for taking physical measurements in a location which is remote from the site of an associated display device, remote from the site of an evaluation device and/or remote from the site of an operating device. An evaluation device and/or operating device is/are namely often located in a central control room or a central control centre. The evaluation device therefore often needs to be supplied with the measurement values generated by the field device over a long distance. Direct wire connections can be used to bridge these long distances. However, instead of the direct wire connections, communications networks can also be used which are arranged between an evaluation device and the field device. The connection to a communications network can be wired or wireless according to the application. The communications network can itself also use any combination of wireless or wired transmission technologies in order to connect at least two remote locations. Both a private and public communications network can be used as the transmission medium.
To access a communications network a wireless transmission technology can be used such as GSM (global system for mobile communications), UMTS (universal mobile telecommunications system), GPRS (general packet radio service) or LTE (long term evolution), mostly according to the chosen communications network. Unlike GSM, UMTS, GPRS or LTE, which are characterised in particular by having a long radio range, wireless access technologies can also be used that are locally more restricted, such as for example WLAN (wireless local area network), often based on a standard of the IEEE-802.11 protocol family, WiFi™, ZigBee or Bluetooth®. An example of wired access technology is the use of a modem connection or a DSL link (digital subscriber line).
However, if a field device or a field device apparatus is prepared for communication via a communications network, said field device is also reliant on the fact that the communications network is substantially always available. If the communications network should fail, it will no longer be possible to transmit data and in particular measurement values.
A mobile end device or mobile user equipment (UE), for example a smartphone, can comprise a tether mode in which a connection is made between a smartphone and a PC (personal computer) to enable an internet connection for the PC via GSM/UMTS. The mobile end device thus takes on the function of a modem.